Cardiac arrythmias are frequently treated with electrophysiology procedures and ablation. In the U.S., Europe and Japan during the last 20 years most of these procedures have been performed endocardially (inside the heart) with catheters that have been navigated from leg and neck veins into the heart chambers. To some degree this is a historical accident. Initially ablation was done primarily for simple supraventricular tachycardias that have their focus on or near the endocardium. However, the fastest growing procedures are those done to treat atrial fibrillation (AF) and ventricular tachycardia (VT), which both have a major component on the epicardium. In fact, the now common pulmonary vein isolation ablation procedure to cure AF, which is done endocardially, actually targets epicardial fibers. Energy is delivered endocardially at a level that is sufficient to penetrate the atrial tissue and destroy the responsible epicardial fibers. However, ablating from inside the heart in order to target the outside tissues has risks, including stroke, and may require more energy than when these fibers are ablated directly from the epicardial side. AF ablation is now performed routinely at academic centers and is entering the mainstream of cardiological practice. It will soon become even more widely used because the American Heart Association recently issued a guideline that upgraded pulmonary vein isolation for AF to a first-line therapy.
Similarly, ablation of ventricular tachycardia (VT), while relatively rare now, is becoming more common as more patients are now surviving heart failure due to implantable cardiac defibrillators and improved medicines. The substrate in VT ablation is epicardial in 23-50% of patients. Ablation for this purpose is typically performed endocardially as well, with the goal of delivering energy transmurally across 10-15 mm of ventricle to destroy fibers epicardially. However, some care centers do perform the ablation epicardially, but there are barriers.
A major barrier to clinical adoption of epicardial ablation is the lack of tools, specifically catheters and sheaths, designed for use specifically in epicardial procedures. While there are a myriad ablation catheters for endocardial ablation, to our knowledge no ablation catheter has been developed specifically for epicardial ablation. Thus, practitioners of epicardial ablation are forced to use endocardial tools despite the many differences in the practice of endocardial and epicardial ablation.
The prior art is silent on the teaching of novel catheters designed specifically for epicardial ablation. The extant prior art that defines the present standard of care therefore has several significant limitations when viewed from the vantage point of the medical devices needed for an epicardial ablative procedure.
In the standard endocardial approach, access to the heart is obtained by inserting a sheath and catheter into the leg veins and then navigating the sheath up the entire vena cava into the right atrium, and then gently curving into the rest of the heart.
Another limitation of the prior art has to do with the geometry of the existing devices. In the endocardial approach there is one only turn that must be made; from the vena cava to the right atrium. This curve is gentle. Other areas of the heart can be reached by moving inside of the chamber of the heart. Thus, there is room to turn and as a result, endocardial sheaths and catheters have only a single adjustable curve. However, in the pericardium there is little room to move, which limits the ability to maneuver. When carrying out a subxiphoid access, there are three separate turns that must be made: (1) A 90 degree turn immediately after entering the pericardial space, (2) a 130 degree (approximate and variable) at the apex of the heart, and (3) a final positioning turn (variable) right at the ablation side.
In conventional practices, for example, these turns are made by pushing and then bouncing a standard catheter against the heart and pericardium in order to force the catheter in the correct direction. This is not ideal since it could lead to injury and might also easily unravel the device during positioning.
Another deficiency with conventional catheters is that no irrigated-tip devices built for epicardial use exist. Animal studies show that irrigated tip catheters make deeper, larger lesions than non-irrigated-tip catheters. In particular externally-irrigated (open) tips, where cool saline is infused near the tip of the ablation catheter, make the deepest lesions. Thus, many centers prefer irrigated tips infusing saline at 17 cc/min in ablation of AF and VT. However, irrigation during epicardial ablation means infusing saline into the limited pericardial space. An infusion of even 100 cc (corresponding to just under 6 minutes of ablation if the infusion rate is 17 cc/min) is enough to cause themodynamic collapse due to pericardial tamponade. Even smaller fluid levels can make epicardial ablation difficult by making the catheter float in a sea of fluid. These are the reasons that Stevenson and Sosa, experts in ablation, have written that externally-irrigated tips are to be avoided epicardially. Sosa E, Scanavacca M. Epicardial Mapping and Ablation Techniques to Control of Ventricular Tachycardia. Journal of Cardiovascular Electrophysiology 16:449-452, 2005; Zei P, Stevenson W. Epicardial Catheter Mapping and Ablation of Ventricular Tachycardia Heart Rhythm 3:360-363; 2006.
Another limitation of conventional ablation catheters is that they provide energy in all directions. As a result, anything continually touching the catheter while ablating is damaged. Inside the heart this is reasonable since one side of the catheter is placed against the target heart tissue and the rest of the catheter is surrounded by moving blood. Thus the fixed target is burned but the moving blood is not damaged. Epicardially, however, there is no moving blood. Thus, when placing the catheter against the target the rest of the catheter may touch adjacent structures such as esophagus, lung or phrenic nerve, which are all stationary structures that could be damaged.
Thus, an aspect of the present invention epicardial catheter shall be the ability to direct its energy in one direction.
Another limitation in conventional catheters is that the trend has been to make the ablation tip longer. Initially ablation tips were 4 mm in length. They have since grown to 8 mm or, in some cases, even 10 mm long. While these larger tips have been shown to make deeper lesions in the endocardium due to blood cooling the large tip, there is no blood in the epicardium. Furthermore, in the endocardial space the catheter floats in the chamber and only a small part of an 8 mm tip will have contact with the endocardium, and thus the lesion can be specific to where there is contact. However, on the epicardium the pericardium will push the entire tip onto the epicardium and lead to a non-specific lesion. In addition, while larger tips make a larger lesion when the tip is non-irrigating, animal studies have shown that smaller tips make better lesions when the tip is irrigating.
Accordingly, an aspect of an embodiment of the present invention provides a catheter that will have a smaller tip, such as about 2 mm tip.
There is therefore a need in the art for an effective epicardial ablation catheter to provide improved means of treatment with regard to each of the heretofore discussed limitations in the prior art. The present invention provides a solution to the problems currently faced by these limitations. The advantages and features of the invention disclosed herein will be made more apparent from the description, drawings, and claims that follow.